The present invention relates to an efficient method for fire extinguishment of a burning metal and a fire extinguishing agent suitable therefor. More particularly, the invention relates to a method for fire extinguishment of a burning metal which can never be extinguished by pouring water or rather gains headway by pouring water and is hardly extinguishable by sprinkling a conventional fire extinguishing agent as well as to a fire extinguishing agent suitable therefor.
As is known, certain metals are combustible in air and are heavily dangerous when the metal takes fire in respect of the difficulty in extinguishment of the fire. Examples of such dangerous metals include magnesium, aluminum, zinc, titanium, zirconium, iron, rare-earth metals, e.g., neodymium, and the like in a powdery form as well as alkali metals such as sodium, potassium and the like irrespective of the form. The metals of the former group are combustible, especially, in a fine powdery form and, once the powder takes fire, the metal burns violently sometimes to cause serious explosion. When the metal powder is burning and heated at high temperatures, the metal readily reacts with water to produce explosive hydrogen gas. Therefore, pouring of water to a burning metal powder can never be a means of fire extinguishment and must be strictly avoided in order not to cause explosion of the hydrogen gas and the so-called steam explosion by which the metal powder is scattered around to badly spread the fire. Conventional fire extinguishing agents other than water such as carbon dioxide gas and Halons as well as powdery fire extinguishing agents, i.e. so-called dry chemicals, are also almost ineffective for the fire of metal powders. A means barely effective for extinguishment of fire of a burning metal powder is to sprinkle dry sand or a special powdery chemical or a dry powder such as sodium chloride, sodium carbonate and the like by which the fire may be suppressed to some extent if not completely extinguished. The use of such a dry powder is not advantageous in practice because a quite large amount of the powder must be sprinkled and the metal powder heated at high temperatures remains lastingly in the core portion of the powder pile in the form of a red-heated ember which must be kept as such sometimes for 30 to 60 minutes or even longer involving a danger of burning up again depending on the conditions. In addition, it is practically a difficult matter to stock a large amount of sand in an absolutely dry condition.
Alkali metals such as sodium and potassium are still more dangerous than the metal powders of the above mentioned class. These alkali metals, even at room temperature or not in a powdery form, readily and violently react with water to evolve a large quantity of heat to cause melting of the metal and produce hydrogen gas which spontaneously takes fire sometimes to cause explosion. Therefore, these alkali metals must be strictly kept away from contacting with water. Other known fire extinguishing agents are almost ineffective for the fire of alkali metals. Like the powders of the former class metals, a barely effective means for extinguishment of fire of an alkali metal is to completely cover up the burning site of the alkali metal with a large volume of dry sand or dry powder mentioned above to effect the suffocating effect for extinguishment taking a rather lengthy time.
The inventor previously has got an idea that such a metal fire may be efficiently extinguished by sprinkling a powder of high-purity boron oxide almost free from water and conducted extensive experiments by using a boron oxide powder or a blend of a boron oxide powder and a mineral powder such as talc, clay, mica and the like to obtain a promising result. A problem in such a powder or powder blend is that coalition or caking of the particles takes place during storage of the powder to cause a difficulty in sprinkling of the powder. When, for example, a fire extinguisher is filled with the powder and used to eject the powder under a gaseous pressure by opening the valve after storage for a length of time, the ejectability of the powder is gradually decreased in the lapse of time for storage so as to leave a considerable portion of the powder unejected in the fire extinguisher as a consequence of the decreased flowability of the powder due to caking of the powder.